Why does it snow less when it’s colder? How do snowflakes start? Why are snowflakes so different? It all has to do with temperature and humidity.
We know from our distant memories of muggy summer days that warm air holds more humidity, but for it to snow there must be some humidity in the air. All snowflakes start when that humidity (water vapour) freezes and crystalizes around a small piece of dust, pollen or pollution in the air. As this ‘seed crystal’ falls, it grows and becomes hexagonal because of the H2O structure of water molecules. From that original hexagonal shape, complex snowflakes can form. For example, the humidity in the air may cause each point on the hexagon to sprout and branch giving us the classic, lacy six pointed snowflake (stellar dendrite).
With the chart below, you can predict the type or shape of a snowflake based on the humidity and temperature. Perhaps one day we will get a snowflake-shape forecast with our regular winter weather report!
Unfortunately, the lacy beauty of delicately branched snowflakes evaporates quickly, leaving us with granules of ice. This process is called sublimation and is the evaporation of the fine outer parts of the crystal which condense onto the central part of the crystal.
Enjoy the snowflakes while you can!
Check out Kenneth G. Libbrecht's SnowCrystals.com to view some incredible pictures of snowflakes! Kenneth G. Libbrecht is a professor of physics at the California Institute of Technology (Caltech) and studies the molecular dynamics of crystal growth, including how ice crystals grow from water vapor, which is essentially the physics of snowflakes.